Na-based Liquid Metal Batteries: Material Compatibility and Cell Performance

Introduction:
In our study, we focus on all-liquid Na-based liquid metal batteries (LMBs) by coupling the liquid Na negative electrode with the heavy liquid metal alloy SbBi9 as positive electrode. As electrolyte, we utilise the molten salt mixture LiCl-NaCl-KCl (61-3-36 mol%), which combines reasonably low Na solubility and high Na+ ion conductivity. Na-LMB cells with the selected active materials are operated at temperatures around 450 °C.
The higher working temperature of LMBs compared with conventional batteries leads to an increased corrosion of solid cell components. In particular, the corrosivity towards structural materials of the heavy liquid metal alloy used as positive electrode is of most concern. Therefore, specimens from different candidate positive current collector (PCC) materials such as austenitic stainless steel of 316 grade and Mo were exposed to Sb-Bi alloys for 750 h at 450 °C, the envisaged working temperature of the LMB cells. In addition to the static exposure tests, the corrosion of the PCC in an operating cell under cycling conditions was investigated and compared with the static corrosion behaviour.
Besides the material compatibility research, lab-scale Na-LMB test cells with ca. ... mehr5 Ah nominal capacity were built and their electrochemical performance was tested using different characterization techniques. These included charge-discharge cycling with constant current, intermittent charging/discharging tests, and electrochemical impedance spectroscopy.

Results:
Na/LiCl-NaCl-KCl/SbBi9 test cells achieve excellent electrochemical performance in short-term exposure tests. The low self-discharge of only 10 mA (0.35 mA/cm²) leads to a high Coulombic efficiency of >98% when cycled at 100 mA/cm². The cells run stably for more than 700 cycles (55 days) with an energy efficiency of 70-80% and an active material utilization of up to 80%. However, after ca. 800 h (33 days) of operation, the cell performance deteriorates when stainless steel is used as PCC.
Stainless steel shows a 7-9 μm thick corrosion layer after 750 h of static exposure to Sb-Bi alloys. When used as PCC in a battery under cycling conditions, the corrosion is slightly increased. In contrast, Mo shows excellent corrosion resistance in Sb-Bi alloys, both in static and cycling conditions, which results in a more stable long-term cell performance.
From the different cell tests, electrochemical cell parameters such as the ohmic resistance, the charge transfer resistance, and the diffusion coefficient of Na in the positive electrode are determined. Different time regimes of the mass transport overpotential are identified, comprising purely diffusive and convection-enhanced mass transport. The combination of the different electrochemical tests allows to characterize and quantify the various dynamic processes in the cell, spanning the range from milliseconds up to 1000 s.